During the fabrication process for semiconductor wafers, it is common to place one or more test structures on the wafer. Subsequent to the provision of the test structures, one or more critical dimensions, such as width, of the structures are measured, generally using a scanning electron microscope (SEM). If the critical dimensions of the test structures are equal to, or lie within an acceptable range, of pre-defined values for the dimensions, the fabrication process is taken to be operating within acceptable parameters. If the critical dimensions of the test structures are not equal to, or do not lie within the acceptable range, of the pre-defined values, the fabrication process is taken to be operating outside the acceptable parameters, and the process may be stopped.
A number of potential problems exist with the measurement of such critical dimensions. For example, the measurements involve use of an SEM, which is an expensive measuring device. Further, critical dimensions of test structures are measured, not actual structures of the semiconductor wafers.